PART I

Explanatory Models ForPersonality

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Psychophysiological andBiochemical Correlates of

Personality

Robert M. Stelmack and Thomas H. Rammsayer

The degree of activation, as shown by the writer invarious publications (Duffy, 1962), appears to affectboth sensory sensitivity and motor response, and isinvolved in those consistencies of behavior that wecall personality characteristics. (Duffy, 1966: 281)

INTRODUCTION

Considering that these quoted words werewritten by Elizabeth Duffy 40 years ago, theview expressed was prescient indeed. Thereis considerable evidence today, from psy-chophysical, psychophysiological and bio-chemical procedures (formerly consideredmeasures of activation), establishing that thepersonality dimension of extraversion (E) ischaracterized by individual differences insensitivity to simple physical stimulation andin the expression of motor responses. At thetime when Duffy expressed her views, how-ever, the association of personality with sen-sory sensitivity and motor processes was farfrom clear. In fact, in an assessment of the

personality literature, Duffy (1962: 273) concluded that ‘Any survey of physiologicalstudies of personality must recognize the sur-prising fact that relatively few investigatorshave reported relationships of any magnitudebetween physiological measures and measures of behavior within the normal population.’ Since that time, however, therewas considerable progress in delineating reli-able relations between personality traits andphysiological processes. This progress wasabetted by the development of rigorous per-sonality typologies; by compelling, large-scale projects determining the heritability ofpersonality traits; by refinement and develop-ment of physiological measurement proce-dures; and by exploiting new paradigms forprobing psychological processes such as sensation, attention, learning and memorythat are manifest in individual differences inpersonality. In this chapter, we mark thisprogress by assessing the current status of the psychophysiological and biochemicalcorrelates of personality traits.

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The nomenclatural framework for thepresent review consists of the three majorpersonality dimensions of E, emotional stability–instability/neuroticism (N), andpsychoticism (P)/impulsive sensation-seeking(ImpSS). These personality traits emerge asfundamental factors in most major personalitytypologies (e.g. Costa and McCrae, 1992;Eysenck and Eysenck, 1991; Zuckerman,2002) and they capture the bulk of psy-chophysiological and biochemical researchon individual differences in personality. An emphasis in this review is placed on electrocortical procedures (i.e. electroen-cephalography (EEG) and event-relatedpotentials (ERPs)), and biochemical analyses(i.e. dopamine, serotonin, and cortisol),because these measurement procedures predominate in current research on personality.Conclusions drawn from earlier reviews ofresearch on the biological bases of personal-ity are briefly stated. An attempt is made to focus the functional significance of the biological procedures and paradigms onthe social and behavioural expressions thatcharacterize the personality dimensions, butthe theoretical frameworks that inspiredmuch of this research are left to other authorsin this volume.

PSYCHOPHYSIOLOGICAL ANDBIOCHEMICAL CORRELATES OFEXTRAVERSION

In previous reviews, it was concluded thatthere were fundamental differences betweenintroverts and extraverts in their reaction tosensory stimulation and in their expression ofmotor activity (Matthews and Gilliland,1999; Stelmack, 1997). There is compellingevidence from a range of measurement pro-cedures indicating that introverts are morereactive or sensitive to simple sensory stimu-lation than are extraverts. Introverts displaylower absolute auditory sensitivity (e.g.Stelmack and Campbell, 1974), lower painthresholds (e.g. Barnes, 1975), lower noise

thresholds (e.g. Dornic and Ekehammer,1990), larger skin conductance responses tomoderate intensity tones (e.g. Smith, 1983),and larger ERP amplitude to simple physicalstimulation (e.g. Stelmack and Michaud-Achorn, 1985). Moreover, there was evidencefrom brainstem auditory evoked potentialsindicating that these intensity effects are evident at the level of the auditory nerve (e.g.Stelmack and Wilson, 1982). These effectsmeld with the preference of introverts forquiet and solitude (Campbell and Hawley,1982) and with their tendency toward with-drawal as a coping strategy in stressful socialsituations (Endler and Parker, 1990).

Introverts and extraverts differ in theirexpression of motor behaviour on a variety oftasks that require a simple motor response,with extraverts initiating faster and more fre-quent responses than introverts (e.g. Brebnerand Flavell, 1978). These effects appear relevant to the disposition of extraverts toliveliness, activity, and talkativeness(Eysenck and Eysenck, 1975), involvementin athletic activities (Eysenck et al., 1982),restlessness in restricted environments (Gale,1969), and preference for physical activity(Furnham, 1981). Moreover, there was evidence employing psychophysiologicalprocedures that differences in motor activitybetween introverts and extraverts can bereferred to peripheral nervous systemprocesses (Stelmack and Pivik, 1996). Thereis good evidence that variation in dopaminer-gic activity (DA) is an important determinantof differences in E (e.g. Rammsayer et al.,1993). In general, more recent research on E and differences in sensory sensitivity andmotor expression, using electrocortical andbiochemical measurement procedures,endorse these findings.

Extraversion and theelectroencephalograph

The electroencephalograph (EEG), recordingelectrical activity of the brain from small elec-trodes affixed to the scalp, was an important

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method for assessing cortical activity of thebrain in the early study of the ascendingreticular activating system (ARAS; Lindsley,1951) and in exploring the role of the ARASin attention, memory and learning. Thehypothesis that differences in E were deter-mined by differences in cortical excitationand inhibition (Eysenck, 1957) and corticalarousal (Eysenck, 1967) fostered extensiveanalysis of E and the EEG. In early reviews(Gale, 1973; O’Gorman, 1977), support forthe notion that introverts are characterized byhigher levels of cortical arousal (indexed bylower EEG alpha wave activity) thanextraverts was equivocal. These reviews didprompt improvements in design and recordingtechniques in subsequent research. Laterreviews conceded that the direction of theresults of these inquiries is towards higherlevels of cortical activity for introverts(Matthews and Gilliland, 1999; Stelmackand Geen, 1992).

In more recent research, the ambiguoushistory of research on E using EEG recordingis continued rather than clarified. The specificconditions under which reliable effects arereplicated remain indeterminate. Tran et al.(2001) observed greater EEG activity in the8–13 Hz (alpha) frequency range forextraverts than introverts but only at frontalelectrode sites. This contrasts with other positive reports (e.g. O’Gorman and Lloyd,1987) showing greater EEG activity at poste-rior electrode sites where alpha activity ismaximal. In a project similar to Tran et al.(2001), higher E was associated with greateractivity in low-frequency EEG bands (deltaand theta) at temporal and parietal sites, andlower alpha activity at temporal and frontalsites (Knyazev et al., 2002). In another well-executed project, no EEG effects for E wereobserved (Schmidtke and Heller, 2004).Notably, the functional significance of theEEG effects in the studies cited here, whenthey are observed, is opaque. Typically, theEEG recordings were obtained while partici-pants opened and closed their eyes. Withoutsome experimental manipulation, few infer-ences of the functional significance of the

EEG can be made. An exception here is thework by Knyazev et al. (2002), where partic-ipants performed mental arithmetic duringthe EEG recording in an attempt to manipu-late arousal level. Even in this case, however,the behavioural effect of this manipulationwas not measured.

There was considerable interest in theclaim that activation of right anterior corticalareas is associated with the expression ofnegative affect, whereas activation of leftanterior cortical areas is associated with theexpression of positive affect (Davidson andFox, 1982). Investigation of these effects wasdrawn into the personality domain byHagemann et al. (1999) who exploited theassociation of E with positive affect and N with negative affect (Tellegen, 1985).Contrary to expectations, higher negativeaffect scores were associated with greateractivation at left anterior temporal corticalareas. As Hagemann et al. (1999) note, thisresult is typical of the mixed outcomes thatplague EEG research on emotion and mood.No differences in EEG activity betweenintroverts and extraverts were observed.

The line of inquiry initiated by Hagemannet al. (1999) was pursued by Gale et al.(2001). During EEG recording, participantswere asked to empathise and rate photo-graphs expressing positive and negativeaffect. Negative valence photographs elicitedgreater activation at left frontal cortical sites,an effect that endorses the sensitivity of theEEG measures to the affect manipulation.Robust effects were reported with extravertsexhibiting greater alpha activity at frontal,temporal and occipital sites.

Gale et al. (2001) state that their dataaccord with the view that extraverts are char-acterized by lower levels of tonic arousal asproposed by Eysenck (1967). Alternatively,one could argue that introverts were morereactive to the photographic stimuli thanextraverts, a view concordant with an extensive literature showing the greater sensi-tivity of introverts to sensory stimulation ingeneral (Stelmack, 1990). The positive andnegative valence photographs did not exercise

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interactive effects on E; that is, one wouldsuppose that the positive affect inductionwould favour the extraverts, resulting ingreater frontal left hemisphere cortical areas.Overall, when EEG is recorded under restingconditions or with minimal or uncontrolledstimulation, the studies cited provide littleconsistent evidence associating E with greateralpha activation.

Extraversion and event-relatedpotentials

Event-related potentials (ERPs) are recordsof the electrocortical activity in the brain thatis evoked by physical stimuli and modulatedby psychological processes such as attention,memory and cognition. ERPs are derived byaveraging ongoing EEG activity that is time-locked to specific stimulus events. It isassumed that random EEG activity emanatingfrom neural sites that are not engaged in therepeated presentation of the stimulus is can-celled out in the averaging. What remains isa signature of the neural activity that occurredduring the processing of the stimulus. Thissignature is a result of the initial activation ofperipheral nerves and nuclei in the brainstemand of the subsequent sequence of neuralactivity along cortical projection pathways.

Extraversion and sensory ERPsEarly research on E and ERPs examinedwaveforms that were elicited by simple sen-sory stimuli such as brief light flashes orsimple tones. Initially, inconsistent effectswere reported that yielded to replicableresults as the conditions for favourable find-ings became apparent. In ERP waveforms totones, larger amplitude for introverts thanextraverts is observed with some consistencyfor ERP waves that develop 100–200 ms fol-lowing stimulation, notably when stimuli are(1) moderately intense, (2) lower frequency,and (3) presented in mixed serial order(Bruneau et al., 1984; Stelmack andMichaud-Achorn, 1985). These effects,which account for about 10% of the variation

in E, are congruent with the greater responseto stimulation in introverts than in extravertsobserved with psychophysical and auto-nomic system measures. Subsequently, therewere few attempts to examine E and ERPusing systematic changes in stimulus inten-sity or frequency. Occasionally, however, theenhanced response to auditory stimulation isobserved incidentally (e.g. Doucet andStelmack, 2000).

Extraversion and brainstemauditory evoked responsesA number of authors explored differencesbetween introverts and extraverts by record-ing brainstem auditory evoked responses(BAER). BAER waveforms capture electri-cal activity along the auditory pathway thatdevelops within the first 10 ms of acousticstimulation. The neural generators of thesewaves, the auditory nerve (wave I), cochlearnucleus (wave II), lateral lemniscus and inferior colliculus (wave V), are well docu-mented. The shorter BAER wave V latencyfor introverts than extraverts is the effectmore consistently observed (Bullock andGilliland, 1993; Stelmack and Wilson, 1982;Swickert and Gilliland, 1998). A recentreport from Gilliland and colleagues is perhaps the most definitive (Cox-Fuenzalidaand Gilliland, 2001). Introverts exhibitedshorter wave V latency than extraverts, withcorrelations in several analyses ranging fromr = 0.23 to 0.28. Gender effects, which areknown to influence BAER latency, were notaccounted for in these analyses. On thewhole, the effect sizes were comparable tothe marginally significant effects withsmaller sample size reported by Stelmack et al. (1993a).

Although effect sizes tend to be modest,accounting for less than 10% of variation inE, the shorter wave V latency for introvertsthan extraverts is a reliable effect that is con-sistent with the greater reactivity to physicalstimuli of introverts observed with othermeasures. The BAEP is exquisitely sensitiveto changes in stimulus intensity with higherintensity stimulation evoking shorter latency

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and larger amplitude BAEP waves. Becausecollaterals from the auditory tracts ascendingthrough the brainstem innervate the ARAS,the amygdala and the cortical centres, the BAEP effects do endorse the arousalhypothesis as noted by Matthews andGilliland (1999), and also the view espousedby Woodward et al. (2001) concerning therole of the amygdala for highly reactive chil-dren. From a neurophysiological perspective,however, the inhibitory influence of theolivocochlear nucleus on brainstem nuclei isreduced or absent for intensities above 75 dBand these inhibitory effects are independentof the reticular system (Desmedt, 1975).Thus, the BAEP effects cannot be understoodin terms of a corticoreticular loop as adoptedby Eysenck as the basis for individual differ-ences in E. The independence of BAEPwaves from descending inhibitory effects isunderscored functionally by the remarkableinvariance of BAEP waves during differentstages of sleep and arousal (Campbell andBartoli, 1986) and even during metaboliccoma (Chiappa, 1990). Similarly, the weightof the evidence indicates that BAEP wavesare not influenced by directed attention(Connolly et al., 1989; Picton et al., 1981).

Extraversion and P3The P3 wave is a positive ERP wave thatdevelops maximum amplitude at about 300 msin simple decision tasks. This wave is usefullyexploited in cognitive psychology, to studyattention, memory and decision making. In general, the latency of the P3 is widelyaccepted as a measure of stimulus evaluationtime that is independent of response selectionand execution processes (Kutas et al., 1977).The P3 wave decreases in amplitude withincreases in task difficulty and can be parsi-moniously understood as an index of pro-cessing capacity (Kok, 2001). Severalinvestigators examined individual differ-ences in E during an auditory oddball task where a P3 wave develops to deviantstimuli presented among a series of standardstimuli. The most consistent effect is largerP3 amplitude for introverts than extraverts

(Brocke et al., 1996; Daruna et al., 1985;Ortiz and Maojo, 1993; Polich and Martin,1992; Wilson and Languis, 1990). Similarly,smaller decrements in P3 amplitude acrosstrial blocks for introverts were reported(Ditraglia and Polich, 1991), although oppo-site effects were subsequently observed(Cahill and Polich, 1992). Null effects werereported by Pritchard (1989). In early work,the larger P3 amplitude for introverts wouldbe attributed to differences in the amount ofresources allocated to the processing of thedeviant stimuli. Other interpretations of theeffects are possible, for example differencesin processing capacity, or even differences insensitivity to stimuli. There is some evidencethat P3 is larger to more intense stimuli (e.g.Gonsalvez et al., 2007). The understandingof these P3 differences is hampered becausethe effects have not been put to the test of direct manipulation or concomitant behavioural evaluations.

Individual differences in E and P3 ampli-tude and latency were also explored in several decision-making paradigms. The out-comes of this work were equally varied.Introverts displayed larger P3 amplitude thanextraverts during a difficult visual vigilanceoddball task (Brocke et al., 1996). Brocke et al.(1997) subsequently observed this effectunder quiet conditions, but extraverts exhibitedlarger amplitude than introverts when thetask was performed during noisy conditions.A larger P3 amplitude for extraverts was alsoobserved in a visual classification task(Stenberg, 1994). More recently, larger P3 amplitude for extraverts was observed tohigh intensity target tones in an auditory odd-ball task (Guerrera et al. 2001). No differ-ences in P3 amplitude between introverts andextraverts were reported in several studiesusing a series of elementary cognitive tasks(Stelmack et al., 1993b), simple response andstimulus-response compatibility tasks (Doucetand Stelmack, 2000), or difficult targetrecognition tasks (De Pascalis, 1993).

The larger P3 amplitude for introverts thanextraverts to moderate intensity target tonesduring an auditory oddball task was observed

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with sufficient consistency to regard it as avalid effect that accounts for about 10% ofvariation in E. How the effect is interpretedand what it contributes to our understandingof E is not yet decided. In general, the effectis congruent with the greater electrodermalresponse amplitude for introverts observed inorienting response paradigms. These effectsare regarded as intensity effects reflecting thegreater sensitivity to stimulation of introverts.A systematic investigation of the effects ofintensity on P3 is clearly desirable to assessthat hypothesis. The larger P3 amplitude forextraverts observed in some studies is a puz-zling effect that also requires more intensiveinvestigation to disentangle sensory andmotor contributions. There is little evidencelinking E to differences in P3 amplitude onelementary cognitive tasks. Moreover, thereis scant evidence of differences in P3 latencythat would link E to differences in cognitiveprocessing speed.

Extraversion and lateralized readiness potentialsThere is a copious literature that implicatesdifferences in the expression of motor behav-iour as a fundamental determinant of differ-ences in E (e.g. Doucet and Stelmack, 2000).These differences in motor expression wereexamined using simple response time (SRT)measures. Although faster and more frequentresponding for extraverts was frequentlyobserved, null effects were also reportedoften. Some progress in clarifying the dispar-ities in this SRT work involved distinguishingbetween response decision time (DT), the timefrom stimulus onset to the release of thehome button; and movement time (MT), the time from the release of a home button tothe subsequent press of a target button.

In early research using response timemeasures with elementary cognitive tasks(Stelmack et al., 1993), an associationbetween E and individual differences in MTwas observed, but not in DT. In subsequentwork, MT was manipulated directly by vary-ing the response button distance and byexamining the interactive effects of stimulus

and response compatibility (Doucet andStelmack, 2000). Extraverts displayed fasterMT than introverts under all conditions. Thepattern of results also suggested that theeffect reflected differences in the initiation ofmovement rather than in the acceleration ofmovement from the home button to the targetresponse button. Because there were no indi-vidual differences in DT or P3 latency andamplitude, these effects implicate peripheralmotor processes as determinants of E ratherthan central cortical mechanisms mediatingsensory discrimination or stimulus evaluation.This question was explored in studies thatemployed an ERP measure termed the later-alized readiness potential (LRP).

The LRP is an ERP measure that permitsdirect assessment of movement initiationprocesses following stimulus-related process-ing. The LRP is derived by recording ERPsfrom electrodes placed over the motor areasof the left and right cortical hemispheres.Responses initiated by the left and right handelicit greater electrical activity in the con-tralateral hemisphere. ERPs derived from thesame side as the overt motor response aresubtracted from the ERP of the contralateralhemisphere. When these difference wavesare averaged across hands, they yield theLRP, reflecting pure hand-related ERP asym-metry. Analysis of the interval between theonset of the stimulus and the onset of theLRP (stimulus-linked LRP) is a measure forthe duration of pre-motor activity, includingstimulus analysis, response preparation andsome aspects of response selection. In con-trast, analysis of the interval between theonset of the LRP and the onset of the behav-ioural motor response (response-linked LRP)is a measure of the duration of motor activityindependent of stimulus processing. There isa consensus that the LRP is generated in theprimary motor cortex (Coles, 1989). A patternof greater activity in the response-linked LRPfor extraverts than introverts and no differ-ences in stimulus-linked LRP or P300 latencyand amplitude would confirm the involvementof primary cortical motor processes as rele-vant determinants of individual differences

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in E rather than central cortical mechanismsthat are involved in sensory discrimination orstimulus evaluation.

Rammsayer and Stahl (2004) obtainedLRPs in an auditory two-choice go/no-gotask. With this task, longer response-linkedLRP latencies were found for introverts thanextraverts indicating faster speed of motorprocessing in extraverts than in introverts.There were no E differences, however, forstimulus-linked LRP latencies. The failure todemonstrate a difference in stimulus-linkedLRP latencies was attributed to the low taskdemands induced by the auditory task. In asecond study (Stahl and Rammsayer, 2004),a complex discrimination task was applied toincrease pre-motor, cognitive task demands.With this condition, stimulus-linked LRPlatencies were shorter for introverts thanextraverts, indicating faster pre-motor infor-mation processing for introverts. However,there were no differences in response-linkedLRP latencies, a failure attributed to theabsence of a no-go condition (Stahl andRammsayer, 2004).

Extraversion and dopamine

Dopaminergic (DA) projections from mesen-cephalic cell groups are divided into twofunctionally distinct systems, the mesostri-atal and the mesolimbocortical (e.g. Robbinsand Everitt, 1995). Mesolimbcortical DA isimportant in locomotor activity, active avoid-ance, incentive/reward motivation, associa-tive learning and working memory (Kimberget al. 1997; Müller et al., 1998; Robinson andBerridge, 2000; Salamone, 1994; Sokolowskiet al., 1994; Tzschentke, 2001). MesostriatalDA neurons serve to inhibit and modulate thestriatum (Björklund and Lindvall, 1986),which in turn exerts a powerful inhibitoryeffect on the thalamus and the reticular for-mation (Carlsson and Carlsson, 1990). Anyincrease in mesostriatal DA activity counter-acts the inhibitory effect of the striatum,resulting in increased reticular arousal and,for example, enhanced sensory sensitivity.

From this perspective, differences in DAbrain mechanisms between introverts andextraverts may mediate the greater sensorysensitivity in introverts compared toextraverts (Rammsayer, 2004).

Rammsayer et al. (1993) addressed thequestion, ‘Does pharmacologically induceddecrease in brain DA activity differentiallyaffect the transmission of sensory input intomotor output in introverts and extraverts?’After pharmacological blockade of DA syn-thesis by means of alpha-methyl-para-tyrosine(AMPT), both DT and MT were markedlyimpaired in introverts but not in extraverts ona choice reaction time task. While DTindexes cognitive processes such as stimulusevaluation and response selection that aremediated by the mesolimbocortical DAsystem (Cohen and Servan-Schreiber, 1992;Rammsayer and Stahl, 2006), MT is a validindicator of motor execution that is primarilymediated by mesostriatal DA activity(Amalric et al., 1993; Dunnett and Robbins,1992; Salamone et al., 1993).

Because AMPT produced a non-specificdecrease in DA activity, the D2 receptorblocker remoxipride was chosen in a subse-quent study to selectively affect homeostasisof dopaminergic transmission (Rammsayer,1998). Remoxipride primarily inhibits neu-rons of the mesolimbocortical DA system. Inintroverts, remoxipride caused a reliableincrease in DT compared to extraverts, whileMT was not affected in either group. Takentogether, these findings indicate that intro-verts are more responsive to pharmacologi-cally induced changes in D2 receptor activitythan extraverts, irrespective of the specificDA system involved.

Although there are interactions betweenneurotransmitter systems, the observed dif-ferences between introverts and extraverts inthe transmission of sensory input into motoroutput seem to be a clear function of DAmodulation (Rammsayer, 2003). Depue andCollins (1999) argued that the mesolimbo-cortical DA system is the neurobiologicalsubstrate that mediates E and resulting in differences in incentive-facilitated behaviour.

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Although their model is based on an integra-tion of behaviour, affect and both cortical andsubcortical neural mechanisms, it still lacksdirect corroborative evidence from humanpharmacopsychological studies (cf. Lawrenceet al., 1999).

Following the model of Depue and Collins(1999), Wacker et al. (2006) combinedbehavioural and EEG measures with pharma-cological treatment. As predicted, they foundthat the agency facet of E modulated theeffect of 200 mg of sulpiride, a D2 receptorblocker, on behavioural and EEG measures.However, because dose-dependent pharma-cological effects of sulpiride are unclear, (cf.Rammsayer, 1997), that effect is not definitive.

Using single photon emission tomography(SPECT), Gray et al. (1994) found no associ-ation between D2 receptor binding and E. In two subsequent PET studies (Breier et al.,1998a; Farde et al., 1997), a positive correla-tion was reported between D2 receptor den-sity and E. Similar studies (Breier et al.,1998b; Kestler et al., 2000), however, failedto observe this relation. These inconclusivefindings appear indicative of a complex relation between D2 receptor density and E.

In these PET studies, participantsremained passive during the recording.Fischer et al. (1997), however, presentedtheir subjects with videotaped scenes of indi-viduals walking in a park during the PETrecordings. Enhanced activity for introvertscompared to extraverts in brain areas associ-ated with the mesostriatal DA system wasobserved. This finding endorses DA as abasis for differences in E and accords withgreater DA responsiveness for introverts thanextraverts proposed by Rammsayer (1998,2003; Rammsayer et al., 1993). For Fischeret al. (1997), the visual stimulation may havebeen the critical condition for elicitingincreased mesostriatal DA activity for introverts. In the absence of experimental orpharmacological manipulation, mesostriatalDA activity for introverts and extraverts arewithin a similar range (Rammsayer et al.,1993) and thus no differences in E areexpected under passive conditions.

Genetic factors that may influence E andcause variations in DA were also explored.Benjamin et al. (1996) and Ebstein et al.(1996) reported differences in E and thetype-4 dopamine receptor (DRD4) gene.Numerous subsequent studies both supported(Benjamin et al., 2000; Ekelund et al., 1999;Noble et al., 1998; Okuyama et al., 2000;Ono et al., 1997; Strobel et al., 1999;Tomitaka et al., 1999) and failed (Burt et al.,2002; Ekelund et al., 2001; Gebhardt et al.,2000; Jönsson et al., 1997, 1998, 2002; Kuhn et al., 1999; Mitsuyasu et al., 2001;Persson et al., 2000; Pogue-Geile et al.,1998; Soyka et al., 2002; Strobel et al., 2002,2003b; Vandenbergh et al., 1997) to supportthese findings.

The failures to replicate an associationbetween DRD4 polymorphism and E wasattributed to the use of different questionnairesfor personality assessment, methods thatinflate the potential for false positive results,lack of statistical power, lack of control forethnic variability, or demographic differencesamong the studies participants (cf. Burt et al.,2002; Malhotra and Goldman, 2000; Strobelet al., 1999). None of these factors convinc-ingly justify the failures to replicate the posi-tive findings. Overall, the large number of nullresults challenges the significance of DRD4polymorphism as a biological basis of E.

Although Noble et al. (1998) reported apositive association between the D2dopamine receptor gene (DRD2) and highnovelty seeking, other studies failed to showsuch an association (Burt et al., 2002; Cruz et al., 1995; de Brettes et al., 1998; Gebhardtet al., 2000).

Extraversion and cortisol

Cortisol is a corticosteroid hormone producedby the adrenal cortex with widespreadactions that help to restore homeostasis afterstress. Cortisol levels show a circadian rhyth-micity, with peak values found in early morn-ing and lower levels in the evening. UnlikeN, E does not appear to be associated with

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variability in early morning salivary cortisollevels (Munafò et al., 2006). There is also noevidence for a relationship between E andcircadian cortisol rhythm or basal and stimu-lated free cortisol concentrations (Roy, 1996;Schommer et al., 1999; Zobel et al., 2004).However, a significant correlation between E and plasma levels of cortisol in the earlyafternoon was recently reported (LeBlancand Ducharme, 2005).

PSYCHOPHYSIOLOGICAL ANDBIOCHEMICAL CORRELATES OFNEUROTICISM

In personality classification schemas, such asthe Eysenck Personality Questionnaire(Eysenck and Eysenck, 1991) or the NEO-PI(Costa and McCrae, 1992), N is an emotionalstability–instability dimension that assessesdifferences in mood swings, negative affect,worry and tension. N is an important predictorof stress management, interpersonal effec-tiveness, and the development of clinical disorders involving anxiety, depression andhostility (Zuckerman, 2005). Accordingly, N was the focus of intensive investigationwith psychophysiological procedures andbiochemical assays.

Many of the early psychophysiologicalstudies that examined differences in E alsoexamined differences in N. However, signifi-cant effects for N were seldom reported instudies where simple physical stimulationwas the principal variable manipulated(Fahrenberg, 1987). Psychophysiologicalmethods that record electrodermal, cardiacand electrocortical activity are especiallysensitive to changes in stimulus intensity.The dearth of psychophysiological effects ofphysical stimulation for N suggests that sensitivity to stimulation is not a determinantof differences in N. This view is endorsed by the paucity of evidence linking N to differences in sensory thresholds, painthresholds or noise thresholds, and the psychological reports of those processes.

The vulnerability of N to negative valencestimulation and to stress (notably socialstress such as ego threat) that was frequentlydemonstrated was confirmed with both psychophysiological methods and with biochemical assays.

Neuroticism and the EEG

In a 1981 review that spanned 45 years ofresearch, Gale cited 29 EEG investigations ofpersonality that assessed the relation of EEGindices to E. Overall, the conditions underwhich the recordings were made werebenign. They were better suited to examinethe psychophysiological bases of differencesin attention and arousal that characterise E than hypotheses linking N to differencesemanating from limbic activity. None of thestudies cited in that review reported signifi-cant associations with N. Subsequent studiesusing improved technology to deriveabsolute indices of EEG power reported thesame null effects for N (Matthews andAmelang, 1993; O’Gorman and Lloyd,1987). However, Ivashenko et al. (1999) didassociate higher N with greater beta activityin right temporal areas.

Stenberg (1992) manipulated affectivedemands with conditions involving neutral,pleasant and unpleasant imagery and exam-ined absolute indices of EEG activity forindividuals differing in N. Higher anxietyscorers (i.e. high N and low E) exhibitedgreater theta activity at right frontal sites thanlower anxiety scorers across all conditions,an effect indicative of higher overall emotion-ality. The high anxiety group also exhibitedgreater beta activity in the temporal regionduring the unpleasant imagery condition.Similar effects were observed in a study thatmanipulated arousal level by engaging participants in a mental arithmetic task that isknown to pose an ego threat (Knyazev,2002). Higher N was characterized by higherbeta and gamma activity in frontal regions,and lower delta and theta activity in temporal,parietal and left frontal areas.

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Several authors explored the relationshipbetween EEG asymmetry measures and N scales. Asymmetry measures are obtainedby subtracting left hemisphere EEG powerfrom right hemisphere EEG power. In themain, this work stemmed from research onemotion by Davidson (1993) and colleagues.In their schema, greater left frontal EEGasymmetry is implicated in the experience ofpositive affect and right frontal EEG asym-metry is implicated in the experience of neg-ative affect. Given the association of N withnegative affect, higher N may be characterizedby greater right frontal asymmetry. Somesupport for this hypothesis was reported bySchmidt (1999) who observed greater rela-tive right frontal EEG activity for individualswho scored higher on a shyness scale. EEGactivity recorded under resting conditionsobserved that higher N was also associatedwith greater relative right posterior activity(Schmidtke and Heller, 2004) and withgreater mid-frontal asymmetry variability(Minnix and Kline, 2004).

Neuroticism and dopamine

Because high N scores are indicative of emotional liability, vulnerability to stress, orproneness to anxiety (e.g. Bolger andSchilling, 1991), N can be viewed as a securitymeasurement of potentially threatening envi-ronmental stimuli (Lee et al., 2005). BrainDA is involved in monitoring activities andalso in cognitive and attentional processes(e.g. Saint-Cyr, 2003). From this perspective,high N may be characterized by higher levelsof brain DA activity that enable more sensitiveor intense reactions to perceived stressors.

Preliminary evidence does suggest a functional relationship between the DA neurotransmitter system and N-related personality traits (i.e., detached or avoidantbehaviour). For example, subjects with the D2 receptor gene haplotype 1 exhibit amore neurotic and immature defence stylecompared with those without haplotype 1(Comings et al., 1995). Two PET studies

(Breier et al., 1998; Farde et al., 1997)revealed a negative association between D2 receptor density and individual detach-ment scores. Another study, using SPECT,yielded a positive correlation between striatalD2 receptor density and N (Lee et al., 2005).Similarly, Kestler et al. (2000) reported thatthe depression facet of NEO-PI N was associated with striatal DA receptor densitymeasured by PET. However, Gray et al.(1994) failed to observe an associationbetween N and D2 receptor binding in thebasal ganglia. Additional support for theinvolvement of D2 receptor mechanisms in N is provided by a molecular genetic studywhere an association between a DRD2 pro-moter variant and measures of detachmentand lack of assertiveness was reported(Jönsson et al., 2003).

Neuroticism and serotonin

N is an important liability factor for thedevelopment of anxiety and depressive disorders (e.g. Kendler et al., 1993). Becauseserotonin specific reuptake inhibitors areeffective in the treatment of depression, neu-ronal mechanisms involved in pre-synapticserotonin reuptake may be implicated in N.Serotonergic activity in the brain, which isinvolved in many affective disorders (Graeffet al., 1996), is mediated by the serotonintransporter gene (5-HTT). The principalfunction of 5-HTT is to remove serotoninfrom the synaptic cleft by returning it to thepre-synaptic neuron where the neurotrans-mitter can be stored for later re-release. 5-HTT expression is particularly abundant incortical and limbic areas engaged in modula-tion of emotional aspects of behaviour(Westenberger et al., 1996). In humans, twocommon alleles, the short and long alleles, ina variable repeat sequence of the promoterregion of 5-HTT were linked to N (e.g. Leschet al., 1996; Sen et al., 2004b). N also mediatedthe association between 5-HTT polymorphismand lifetime major depression (Munafò et al.,2006). Analysis of genotype–phenotype

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relations in healthy volunteers by means ofimaging-genomics studies (Hariri andWeinberger, 2003) endorse an associationbetween 5-HTT polymorphism and N; thatis, increased responses of the amygdala as afunction of the short allele in the linked pro-moter region of the 5-HTT (Hariri et al., 2005).

Numerous studies failed to confirm anassociation between 5-HTT polymorphismand N (e.g. Ball et al., 1997; Deary et al.,1999; Ebstein et al., 1997; Flory et al., 1999;Jorm et al., 1998; Mazzanti et al., 1998;Willis-Owen et al., 2005). Several possibleexplanations for these inconsistent resultswere proposed, namely a small sample size,different methods of personality assessmentand phenotype ascertainment, or populationstratification. Attempts to circumvent thesemethodological constraints, however, alsofailed to form a consensus. Five meta-analyseswere also inconclusive (Munafò et al., 2005;Munafò et al., 2004; Munafò et al., 2003;Schinka et al., 2004; Sen et al., 2004a)

Animal research on the serotonin receptorsubtype 5-HT1A provides converging evidencefor serotonin as a biochemical correlate of N.Anxiety is more pronounced in mice lacking5-HT1A receptors than controls (Parks et al.,1998; Ramboz et al., 1998). Further, 5-HT1A

receptor agonists were effective in the treat-ment of anxiety (e.g. Sramek et al., 1997). A negative correlation between the anxietyfacet of the NEO PI-N scale and cortical 5-HT1A receptor binding potential was alsoobserved in a PET study of healthy volun-teers (Tauscher et al., 2001); that is, high N ischaracterized by lower 5-HT1A receptor density. An association between HTR1A-1019 polymorphism and the NEO-PI-R N(Strobel et al., 2003a) also endorses a rela-tion between allelic variation in the 5-HT1A

receptor and the expression of the anxietyand depression aspects of N.

Neuroticism and cortisol

The hypothalamic–pituitary–adrenal (HPA)axis is a major part of the neuroendocrine

system that controls reactions to stress andregulates mood. HPA dysregulation, as indi-cated by excess cortisol response after HPAstimulation, was identified as an indicator ofdepression (Pariante and Miller, 2001;Plotsky et al., 1998). Given that N is a pow-erful predictor of depression, an associationbetween N and HPA dysregulation is plausi-ble. Both N and HPA dysregulation operate asrisk and vulnerability factors for depression(Holsboer, 2000). High N and HPA dysregu-lation are indicative of less effective copingwith stress, critical life events, and psycho-logical challenges. Several studies exploredthe relationship between these N and HPA.

McCleery and Goodwin (2001) were thefirst to demonstrate differences in HPA regu-lation as a function of N. Specifically, low N exhibited a stronger cortisol response thanhigh N. This effect may be indicative of adown-regulated HPA axis for high N to pre-vent harmful over-activation. Subsequently,Zobel et al. (2004) observed the reverse pattern of cortisol response; that is, strongercortisol responses were positively associatedwith N. Zobel et al. (2004) suggested thatHPA dysregulation may provide a biochemi-cal basis for N and depressive temperament.Higher cortisol levels for high N individualswithout a previous history of depression (e.g.Bridges and Jones, 1968; Portella et al.,2005) provides additional evidence that highN is associated with altered HPA regulation.Overall, however, the relationship between N and HPA is not resolved.

PSYCHOPHYSIOLOGICAL ANDBIOCHEMICAL CORRELATES OFIMPULSIVE SENSATION SEEKING

Research on impulsiveness is a challengebecause it is a complex construct with multiplemeanings. In the Eysenck three factor model,all three factors, E, N and P, relate to someaspects of impulsiveness: venturesomeness isa feature of E, while narrow impulsiveness is a feature of P and N (Eysenck, 2004).

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P also features prominently on an SS factorthat is appropriately termed impulsive unsocialized sensation seeking (ImpSS)(Zuckerman et al., 1988). There is a substan-tial psychophysiological literature thatexplores individual differences in SS and thebiochemical analysis of individual differ-ences in ImpSS has flourished in recent years.

Psychophysiology of sensationseeking

From the psychophysiological literature,three conclusions can be drawn. First, thereis little evidence of individual differences inbase level of arousal between high and lowscorers in SS using measures of skin conduc-tance level, EEG desynchronization, or restingheart rate (Stelmack and Geen, 1992). Thesenull effects negate the proposal that high SSis characterized by low tonic arousal(Zuckerman, 1979).

Second, there is good evidence that highSS scorers react more intensely to stimulationthan low SS scorers under some conditions.High SS scorers exhibit larger skin conduc-tance responses than low SS scorers to novelstimulus items that are relevant to the SSscale (SSS; Zuckerman, 1979), for examplepictures of hang-gliding, marijuana smoking,mountain climbing, and sexual and violentstimuli (e.g. Smith et al., 1986). In general,these effects provide good support for theconstruct validity of the SSS, but providelittle insight into the biological bases of SS.

Third, there are reliable individual differ-ences in SS, accounting for about 10% ofvariation, that are observed in an augmenting-reducing paradigm with visual ERP changesto increases in the intensity of light flashes.Individuals with high scores on the disinhibi-tion subscale of the SSS exhibit an increasein amplitude of an ERP wave (P1, N1) thatdevelops at about 100 ms following stimula-tion. Low sensation seekers exhibit adecrease in amplitude with an increase inintensity of the light flashes whereas highsensation seekers exhibit an increase in

response amplitude (Buchsbaum, 1971;Lukas, 1987). More recent evidence fromcarefully executed studies endorses this view(e.g. Brocke et al., 1999).

The augmenting-reducing effect was considered as evidence supporting the viewthat high SS is characterized by lower tonicarousal, and that stimulation is amplified, orsimple physical stimulation is experiencedmore intensely than in low SS scorers, inorder to raise arousal to an optimal level(Zuckerman, 1979). Alternatively, in theabsence of evidence indicative of differencesin base levels of arousal, it can be argued thataugmenting-reducing is an intensity effect inwhich high SS scorers are less sensitive tostimulation than low SS scorers and that lowSS scorers initiate inhibitory, protectivemechanisms in response to high intensitystimulation that result in smaller responses,(Smith et al, 1989). Coincidentally, it hasbeen shown that a high ImpSS is character-ized by greater pain tolerance, greater E, lesshypochondriasis, higher absolute sensorythresholds (Goldman et al., 1983; Kohn et al.,1982) and smaller P3 amplitude to negativevalence emotional stimuli (De Pascalis et al.,2004). This suggests that high SS scorersmay engage in intense stimulating activities,not to achieve an optimum level of arousal, butbecause they can endure intense stimulation.

Impulsive sensation seeking anddopamine

Zuckerman (1994) proposed the construct ofimpulsive unsocialized sensation seeking(ImpSS) as an independent trait of personality,with Eysenck’s P scale as its strongestmarker (Zuckerman et al., 1988). Accordingto Eysenck and Eysenck (1976), a continuumcan be drawn from normal through psycho-pathic behaviour to psychotic states. In thisview, the biological basis of P is continuousfor healthy individuals and psychoticpatients. Increased DA activity is a promi-nent hypothesis in neurochemical theories of schizophrenia (cf. Davis et al., 1991).

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DA activity can also be expected to vary withP or ImpSS (Pickering and Gray, 2001;Zuckerman, 2005). Overall, there is goodevidence associating DA activity and P

Although it is premature to determinewhether E is more strongly related to brainDA than P/ImpSS, there are a number of DAmediated effects related to P or psychosisproneness rather than to E, e.g. latent inhibi-tion (e.g. Gibbon and Rammsayer, 1999;Lubow and Gewirtz, 1995), negative priming(e.g. Beech and Claridge, 1987; Swerdlow et al., 1995), and pre-pulse inhibition (e.g. Kumari et al. 1997; Simons andGiardina, 1992).

Netter and Rammsayer (1991) adminis-tered the DA antagonist haloperidol and theDA precursor L-dopa to normal subjects andtested them on a reaction time task. Whilehigh SS scorers tended to feel more relaxedand perform better after haloperidol, low SSscorers performed better after L-dopa, effectsindicative of more responsive DA activity inhigh ImpSS scorers (Zuckerman, 1993). A negative relationship between P and D2receptor binding in the basal ganglia wasreported in a PET study by Gray et al.(1994). Because an increase in DA activityresults in down-regulation of post-synapticreceptors, as indicated by a decrease innumber of receptors or post-synaptic receptorsensitivity (Creese et al., 1977), the associa-tion between P and D2 binding is indicativeof increased DA activity for P. Initially, thisconclusion appears congruent with thehypothesis of increased brain DA in schizo-phrenia. However, DA hypothesis of schizo-phrenia predicts enhanced activity in themesolimbocortical DA, whereas the Gray et al.(1994) finding referred to the functionallyindependent mesostriatal DA.

Impulsive sensation seeking andcortisol

An early study by Ballenger et al. (1983)reported that SS was characterized by lowlevels of free cortisol. Subsequent studies

measuring cortisol baseline levels (Gerra et al.,1999) and cortisol response values (Gerra et al., 1998) failed to observe that negativerelationship to ImpSS. More recently, a reliable inverse relation between cortisol andSS was reported for male, but not for femalecollege students (Rosenblitt et al., 2001).

SUMMARY AND CONCLUSIONS

Overall, there was good progress in focusingthe fundamental facts of the psychophysio-logical and biochemical correlates of person-ality. The greater sensory reactivity ofintroverts than extraverts to simple sensorystimulation observed with a wide range ofpsychophysical and psychophysiologicalprocedures is well established. There is alsogood progress in demonstrating differencesin motor expression between introverts andextraverts with psychophysiological proce-dures. The faster movement time forextraverts on simple response time tasks, andthe absence of P3 latency effects (an index ofstimulus processing speed) does point to theinvolvement of peripheral and or/corticalmotor processes as relevant determinants ofindividual differences in E rather than centralcortical mechanisms that are involved in sen-sory discrimination or stimulus evaluation.The application of lateralized readinesspotentials is a promising procedure for articulating the sensory and motor effects.

Biochemical analysis of the DA system,which is involved in the neuroregulation ofsensory input and motor output, is proposedas a biochemical determinant of individualdifferences in E (Rammsayer, 2004).Although biochemical analyses revealed thatDA turnover is the same in introverts andextraverts (Rammsayer et al., 1993), there isgood evidence from different procedures for E differences in responsiveness to devia-tions from the physiological level of DAactivity in the brain, with introverts moresusceptible to changes in D2 receptor activitythan extraverts.

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The disappointing outcome of early psychophysiological research on N, usingsimple physical stimulation, has yieldedmore promising results with some EEG procedures. Although the effect is not conclusively established, the association ofhigher N with greater right frontal EEGactivity was observed in several reports,notably under negative affect conditions.

Biochemical analyses of individual differ-ences in N are equivocal. There is some evidence linking N and D2 receptor mecha-nisms, but this evidence is piecemeal.Analyses of the serotonergic system areinconclusive. Although there is good evidencerelating depression to HPA dysfunction (as indexed by excess cortisol response following HPA stimulation) and although N is an important predictor of depression, no firm association betweeen N and HPAdysfunction is established.

With respect to ImpSS, psychophysiologi-cal research indicates: (1) no reliable individual differences in tonic levels of phys-iological activity, (2) greater response tohighly novel, exciting, or disturbing stimulifor higher SS, and (3) larger response(greater tolerance?) to higher intensity phys-ical stimulation for higher SS. Although farfrom conclusive, there is increasing evidencerelating P/ImpSS to increased or more reactiveDA activity.

The review of psychophysiological andneurochemical research presented in thischapter aimed to focus the biological basis ofpersonality and individual differences. Thearousal construct was central to the earlyexamination of personality from a physiolog-ical perspective. A distillation of that work isincorporated in this review. Over the pastfour decades, the neurosciences providednew findings, constructs, and models in anattempt to improve our understanding of thebiological determinants of behaviour andindividual differences, often without integra-tion of previous effects that were reported.As Matthews and Gilliland (1999) suggested,this scenario may have lead, unintentionally,to an oversimplification of a number of

neurophysiological processes. Both theseconsiderations could contribute to the incon-sistency of effects noted in this review.Clearly, future research must make an effortto exploit reliable effects and to incorporatethem in a paradigm of personality that leads to a meaningful appreciation of howneural processes, neurotransmitters, and hor-mones contribute to individual differences in personality.

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